The present invention relates to an artificial vessel which has a porosity and a compliance approximate to that of a vital vessel and the process for preparation of the same.
In recent days, investigation as to an artificial vessel has proceeded and many artificial vessels have been developed with the progress of vascular surgery. At present, examples of the clinically used artificial vessels for arteries with large diameter of not less than 6 mm are, for instance, DeBakey artificial vessel made of woven Dacron (USCI. Co., Ltd. of U.S.A.) and Gore-Tex (Gore. Co., Ltd. U.S.A.) which is made of an expanded polytetrafuoroethylene (hereinafter referred to as "EPTFE").
Those conventional artificial vessels have pores which communicate between the inside of the vessel and the outside the vessel. When the vessel is grafted into a living body, the outside of the vessel is covered with pseudointima and the pseudointima grows into the communicating pores to cover the inside of the vessel, i.e. the vessel is organized, which prevents the formation of thrombus or occlusion by thrombus and thus makes the artificial vessel stable in the living body. The property that the communicating pores serve to make the artificial vessel organize is referred to as "porosity".
According to Sasajima et al, J. Artif. Organs 12(1), 179-182 (1983), the compliances of those artificial vessels are measured. The results are shown in Table 1.
TABLE 1 ______________________________________ Vessel Compliance ______________________________________ Thoracic aorta of dog 0.749 Abdominal aorta of dog 0.491 Carotid artery of dog 0.356 Double Velour Dacron 0.058 Woven Dacron 0.021 EPTFE 0.028 ______________________________________
As shown in Table 1, the compliances of the conventional artificial vessels are much smaller than those of vital vessels, which causes various problems due to compliance mismatch such as anastomotic punnus hyperplasia long time after the grafting in a living body. Particularly, the conventional artificial vessels cannot be clinically used as an artificial vessel for artery with small diameter of not more than 6 mm because the compliance mismatch remarkably increases to make a patency of the vessel bad. Therefore self-veins are used for vascular reconstructive surgery of coronary arteries or arteries below knees.
In order to solve such compliance mismatch, a process for preparing an artificial vessel of an elastomer which has a porous wall and a compliance approximate to that of a vital vessel is disclosed in U.S. Pat. No. 4,173,689. The artificial vessel is prepared by immersing a mandrel into an elastomer solution, taking the mandrel out of the solution, coating the mandrel with the solution, and immersing into a poor solution such as water to deposit the elastomer on the mandrel. However, the process can only provide a vessel having very small pores on its wall and having a relatively dense structure. Although the compliance of the artificial vessel prepared according to the process disclosed in the U.S. Patent is surely larger than that of the conventional artificial vessel, the compliance is still smaller than that of a vital vessel and is not sufficient. In addition, since it is difficult to coat the mandrel uniformly with the elastomer solution, an artificial vessel having the same properties at all parts cannot be readily prepared.
As mentioned above, in general the smaller the diameter of an artificial vessel for an artery becomes, the more it is important that the compliance of the vessel approximates to that of a vital vessel. Although various efforts have been made to obtain an artificial vessel with a compliance approximate to that of a vital vessel, any artificial vessel having the required compliance has not been realized.
Moreover the preparations of the conventional artificial vessels are complicated, which makes the artificial vessels expensive.
Other than the compliance match, an artificial vessel requires properties such as a property that the suture cannot be readily frayed, a property that the artificial vascular material can be optionally cut to an available length, and a property that any kinking cannot be formed. In case of the woven Dacron or the woven polytetrafluoroethylene, a particular textile technique is required in order not to fray at cut ends, and a particular technique such as fellow processing is also required to avoid the formation of kinking, which makes the preparation complicate and the prices expensive. In case of the EPTFE a complicated preparation is required for extention of the PTFE, which makes also the price expensive.
As a result of the inventor's continuous studies, it has been found that an inexpensive artificial vessel having a porosity and a compliance approximate to that of a vital vessel can be prepared by extruding an elastomer solution having a cloud point from an annular nozzle at a temperature of more than the cloud point, immersing the tubular extrudate in a coagulating liquid to precipitate the elastomer, and at the same time cooling the elastomer solution with the coagulating liquid from the temperature of more than the cloud point to a temperature of not more than the cloud point. At that time the phase of the solution changes. The artificial vessel obtained has a network structure in which there are communicating pores over the entire thickness from the inner surface to the outer surface.